Spiraling energy costs have renewed interest recently in the recovery of energy from a variety of energy sources. In particular, the recovery of thermal energy from chemical plant and refinery process streams has received considerable attention. Multiple sidedraw fractionating columns, such as crude oil distilling columns, catalytic cracking unit main fractionators, and thermal cracking unit fractionators, commonly have one or more circulating reflux systems to recover heat for reuse.
In the typical configuration of these systems, reflux liquid is withdrawn from the column, preferably a few trays below a sidedraw tray, circulated through heat exchangers, and returned to the column at the tray just below the side draw tray. Such liquid is normally referred to as circulating reflux. The column usually (although not always) is instrumented in such a way as to require some liquid overflow from the draw tray into the circulating reflux zone to provide a margin for control action. All sidedraw liquid and tray overflow (if any) represents column vapor that was condensed in condensing zones (either circulating reflux sections or the overhead condenser) higher in the column. These zones, because they are higher in the column, are at lower temperatures, and the heat removed is of lesser value because of its lower temperature. Because of this lower value, a process which upgrades the heat from a low grade heat source to a higher grade source has great economic importance. The invention is such a process.